1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly to a semiconductor device which is capable of high-speed switching and which is well suited for forming multi-level interconnections owing to a small required area and reduced surface roughness.
2. Description of the Prior Art
As is well known, the switching speed of a bipolar transistor is greatly affected by the size of a base-collector junction. It has therefore been proposed to enhance the switching speed by making the size of the base-collector junction small.
FIG. 1 shows an example of such proposed device. A p-type substrate 11 has a surface region in which an n.sup.+ -type buried layer 12 is provided. The surface of the n.sup.+ -type buried layer 12 is formed with an insulating film (SiO.sub.2 film) 17 having a plurality of openings. An n-p-n transistor which consists of an n-type collector 13, a p-type base 14 and an n-type emitter 15 is provided in a predetermined one of the openings. The collector 13 is connected to a collector electrode 10" through the n.sup.+ -type buried layer 12, while the base 14 is connected to a base electrode 10' through a polycrystalline silicon film 18 which is deposited on the insulating film 17.
The bipolar transistor having such structure has the merit that, since the p-n junction between the base 14 and the collector 13 is small as apparent from FIG. 1, the parasitic capacitance is small enough to realize a high-speed switching operation.
Since, however, the polycrystalline silicon film 18 is used as a base lead-out electrode, i.e., the interconnection between the base electrode 10' and the intrinsic base region 14, and an emitter electrode 10, the base electrode 10' and an insulating film (SiO.sub.2 film) 19 are formed on the polycrystalline silicon film 18, comparatively great steps (of approximately 1.5 .mu.m) exist between the electrodes 10, 10' and the insulating film 19, and the collector electrode 10" and the insulating film 17 which are formed directly on the silicon substrate. Therefore, when multi-level interconnections are formed on the steps, the parts corresponding to the steps are liable to break, so that the reliability of the semiconductor device degrades drastically.